082 CHAPTER 28 Synthetic Polymers
Table 28.5 Examples of Alkenes That Undergo Anionic PolymerizationCH 2 CH CH 2 CHClCH 2 CCH 3COCH 3OCH 2styrenevinyl chloridemethyl methacrylateacrylonitrileCHCNAnionic Polymerization
In anionic polymerization, the initiator is a nucleophile that reacts with the alkene to
form a propagating site that is an anion. Nucleophilic attack on an alkene does not
occur readily because alkenes are themselves electron rich. Therefore, the initiator
must be a very good nucleophile, such as sodium amide or butyllithium, and the
alkene must contain an electron-withdrawing substituent to decrease its electron den-
sity. Some alkenes that undergo polymerization by an anionic mechanism are shown in
Table 28.5.The chain can be terminated by a chain transfer reaction with the solvent or by
reaction with an impurity in the reaction mixture. If the solvent cannot donate a proton
to terminate the chain and if all impurities that can react with a carbanion are rigor-
ously excluded, chain propagation will continue until all the monomer has been con-
sumed. At this point, the propagating site will still be active, so the polymerization
reaction will continue if more monomer is added to the system. Such nonterminated
chains are called living polymersbecause the chains remain active until they are
“killed.”Living polymers usually result from anionic polymerization because the
chains cannot be terminated by proton loss from the polymer, as they can in cationic
polymerization, or by disproportionation or radical recombination, as they can in
radical polymerization.
Super glue is a polymer of methyl Because the monomer has
two electron-withdrawing groups, it requires only a moderately good nucleophile to
initiate anionic polymerization. An OH group of cellulose or a nucleophilic group of
a protein can act as an initiator. You may well have experienced this reaction if you
have ever spilled a drop of Super glue on your fingers. A nucleophilic group of thea-cyanoacrylate.CH 2 CH−chain-initiating stepBu Li++ CH 2 CHCNBuCN−the alkene monomer
reacts with a nucleophilechain-propagating steps
propagating sites+ CH 2 CHCNBu CH 2 CHCN−CNBu CH 2 CHCNCH 2 CH−+ CH 2 CHCNBu CH 2 CHCNCH 2 CHCN−CNBu CH 2 CHCNCH 2 CHCNCH 2 CH−